Placed in a coin press, working dies are used to create permanent impressions on the planchets that were produced at the upsetting mill. The coin press actually consists of three basic elements: a die for the obverse image, a die for the reverse image, and a collar that surrounds the dies, forming a coining chamber.
The collar is an important element as it centers the planchet for a perfect strike, prevents the striking pressure from extruding metal or flattening the coin, forms the edge of the coin, and creates fluting or other designs in the edge.
The reverse die is usually the lower die and the obverse die the upper, although there are exceptions to this and some presses operate with the dies moving horizontally rather than vertically. Usually, the lower die remains stationary and the upper die comes down to meet it.
In antiquity, the metal had to be heated prior to being struck to make it soft enough to receive an impression. This is unnecessary today for several reasons:
1. modern coins are thin.
2. the metal for the coin has been annealed (making it more malleable).
3. tremendous pressure is exerted by the coin press.
4. the relief on the coin is very shallow.
The collar is five thousandths of an inch larger in diameter than the dies. When the dies strike, the pressure causes the planchet to expand slightly to fill the chamber. Striking pressures are adjusted for various denominations and metals. If the coin is to be fluted or have some other design along its edge, the collar can act as a third die (the edge pattern having been engraved on the collar's interior surface). A second way to impress a design on the edge of the coin is to use an edge incusing machine, which mechanically presses the design into the edge. Without the collar, the coin would be flatter than normal and noticeably thinner in diameter (known as a"broadstrike" error).
Modern coin presses operate very quickly. In such a press, a die will strike approximately 120 coins each minute. Once the impressions have been made the collar can separate into sections to allow the new coin to be removed. In some presses, however, the lower die will rise after the upper die has struck to push the coin out of the coining chamber from below.
Proof coins are struck in a fashion similar to that of regular coins, although they are produced on coin presses specifically adapted for the purpose. The planchets fed into the machine are highly polished and handled so as to minimize scratches and abrasions. The dies are specially processed so as to create frosted images and the background (field) is polished to a mirror-like finish. Furthermore, the dies are actually buffed during the striking process. Each coin is struck at least twice to bring out every detail.